This invention relates to a frequency converting circuit, and more particularly to a frequency converting circuit with small frequency variation.
In the world today, there are strong demands for various quick, high quality information services. To meet such demands, industry has made remarkable advances in communication technology for information services for data, pictures, sound, etc. Typical implemented examples of these are; multichannel sound broadcasting, teletext, facsimile, still picture broadcasting, high quality TV, satellite broadcasting, etc., which are based on the digital technology. In these examples, the digital coding transmission is employed for the multichannel transmission. On the receiving side, for demodulation, the carrier wave signal is first reproduced based on the baseband signal, and subjected to an appropriate signal processing. In this communication system, the sound signal is modulated by a differential phase shift keying (DPSK) modulation system. The DPSK system is in the category of a phase shift keying (PSK) in which a phase of the carrier is varied according to the baseband signal. The PSK modulation is featured by good efficiency and carrier-to-noise (C/N) ratio. In the DPSK system, a phase of the carrier is used as a reference phase, and the phase of the carrier is shifted from the reference phase by predetermined phase angles. The DPSK system usually employed is a 4-phase or differential quaternary phase shift keying (DQPSK) system. In the DQPSK system, the phase of the carrier is stepwise varied in four phases 0.degree., 90.degree., 180.degree., and 270.degree. with reference to the digitized sound signals. On the transmission side, the digitized sound signals are summed before being modulated, while on the receiving side, the received signals are subjected to subtraction for demodulation. The code system for the digitized signals is the so-called Gray code system. On the receiving side, a phase locked loop (PLL) circuit is used for the baseband signal selection.
In the DQPSK system, if noise is superposed on the transmitted DQPSK wave, a vector of the DQPSK wave varies to go out of a reference phase plane of the demodulation phase, and the demodulation axis of the adjacent phase is subjected to demodulation. Such an improper demodulation is also caused by a frequency variation occurring in the frequency converting circuit used for carrier reproduction. This provides a serious problem.
The frequency converting circuit performs the frequency conversion twice to obtain the sound DQPSK which is to be demodulated. The satellite television broadcasting system, for example, has one hundred and several tens channels, each channel having a 6 MHz frequency band. One of these channels is assigned to the exclusive use of the sound. The channel exclusively used by the sound is further divided into a plurality of frequency bands for transmitting the corresponding number of sound signals. To pick up a desired sound signal, the 6 MHz sound channel is first caught and then the desired sound signal is sought. It is for this reason that the two frequency conversions are conducted. This demodulation process will be given in more detail. An input signal of a frequency is applied to a first frequency converter as a mixer. In the first frequency converter, the input frequency is converted into an intermediate frequency, by using the oscillating frequency supplied from a voltage controlled oscillator (VCO) as a part of the PLL circuit. The output signal of the first frequency converter is applied to a second frequency converter. In the second frequency converter, the output frequency of the first frequency converter is converted into a desired frequency by using the oscillating frequency supplied from a local oscillator. This output signal of the second frequency converter is a baseband signal.
The PLL circuit contains a crystal oscillator providing a highly stable or fixed frequency signal. The output signal from the crystal oscillator is phase compared with the output signal from the VCO. The comparison result is used for controlling the VCO operation.
The circuit constants of the oscillator of the PLL circuit and the local oscillators are hardly fixed with time, inevitably resulting in a variation in the frequency and phase. The concomitant frequency and phase variation greatly influence the occurrence of bit error when the sound data is reproduced.
The baseband signal as obtained by frequency converting the input signal is synchronously detected by the DQPSK demodulator. The synchronous detection is a kind of phase detection in which a multiplying circuit multiplies the modulated signal by a reproduction carrier synchronized with the modulated signal, and the product signal is passed through a low pass filter, to obtain phase data of the baseband signal. To demodulate the digital sound signal as transmitted in the form of the DQPSK signal as a 4-phase DPSK signal, the 2-axis synchronous detection is used to detect the data to be Gray coded. To prevent bit error occurrence during this data detection, it is required to minimize a frequency variation when the input frequency is converted into the baseband frequency.
In the conventional frequency converting circuit, the input frequency of the DQPSK demodulation depends on the stability of the crystal oscillator of the PLL circuit and the local oscillators. Assuming that the frequency variation of the crystal oscillator is 50 ppm, and the output frequency of the first frequency converter is 300 MHz, then its frequency variation is large, 15 KHz in frequency. The accuracy of the data transmission is several hundreds of Hz. Therefore, the conventional frequency converting circuit inevitably involves demodulation error. Further, when the conventional frequency converting circuit is used for the carrier reproduction means of the DQPSK demodulation, the frequency pull-in range is narrow. For this reason, even if the frequency is pulled in in the critical region of the pull-in, when data is demodulated by the DQPSK, the demodulated output level is low, and data is likely to be demodulated in the phase not to be demodulated, leading to bit error occurrence.
Assuming that the amplitude of the synchronous detection output signal is .+-.V, and it is superposed with Gaussian noise of variance a.sup.2, a bit error rate Pe is expressed, using a normal error integration function .PHI.(Z) ##EQU1## where C/N is a subcarrier power-to-noise ratio. If the pull-in range is narrow, the C/N is poor and hence the bit error rate is large.
This implies that when the pull-in range is narrow, the pull-in is performed, but the eye aperture is degraded, leading to occurrence of bit error.